1. Field of the Invention
This invention relates to explosion protection systems, such as explosion suppression and explosion isolation equipment. It is particularly concerned with nozzle structure for improved outflow of a liquid or powdered suppressant upon release of the suppressant from a contained supply thereof.
2. Description of the Prior Art
Explosion protection systems gained widespread commercial acceptance after World War II following intensive efforts to suppress combat aircraft fuel tank explosions. Prior to this work, it was widely believed sensing of an incipient explosion could not be accomplished at a fast enough rate to prevent the event from occurring.
Research undertaken in England during the war confirmed that the pressure rate rise in an explosion followed a parabolic type curve. It was demonstrated that if the pressure increase could be adequately sensed during the initial, flatter part of such curve before the pressure started to increase rapidly and exponentially, there was a possibility of preventing an explosion provided the suppressant medium could be delivered to the threatened site during the very first part of the pressure buildup.
Initial studies were directed toward extremely rapid sensing of the pressure rise associated with an incipient explosion. Pressurized detectors capable of detecting a pressure rise within 50 milliseconds after initiation of an event leading to an explosion were soon developed but equipment capable of delivering a suppressant medium to the threatened area within a required short time interval lagged behind detector technology. Pressure detectors now are capable of detecting a 0.25 psi increase in pressure in a time interval of no more than about 2 milliseconds.
Forty years later, efforts are still being made to solve the multiplicity of problems which have been discovered relating to release and delivery of a suppressant medium after detection of a pressure rise indicating that an explosion is about to occur. Many studies have focused on the means to effect release of a suppressant upon command after a pressure rise has been detected. Mechanical valves were first used but they proved to be too slow in operation and also suffered from leakage defects over the long static storage periods required for explosion suppression systems. Equipment of this type must sit in a dormant state and perhaps never operate, but when called upon to do so, function without fail in the required ultra-short reaction time.
The most successful release devices for explosion protection systems have been rupture discs hermetically sealing the delivery opening of a suppressant containment vessel. A shock wave producing device such as an electrically operated explosive initiator or detonator is mounted in proximal relationship to the rupture disc to effect rupture thereof upon receipt of an operating command from a sensing device such as a pressure rise detector.
The functionality and reliability of explosion protection systems has significantly benefited from the use of fluorinated hydrocarbons (e.g., du Pont's HALON 1301), not only from the standpoint of the ability of the HALON to quickly suppress an incipient explosion, but equally as important, because the gaseous product has a low toxicity level. Thus, a system for protecting areas from explosions may be designed even where people occupy the locale.
Explosion protection systems which utilize HALON as the suppressant medium normally embody a containment vessel for confining the suppressant as a liquid under pressure. The HALON is generally held under an applied pressure of compressed gaseous nitrogen also contained in the vessel at a pressure of about 360 psi causing the suppressant to be maintained at its vapor pressure of about 200 psi.
In a typical system, an elongated suppressant delivery or discharge pipe or elbow is connected to the spherical, cylindrical or other pressure containing vessel which is used to store the suppressant medium until a release command is received from a pressure detector or other device capable of sensing the onset of an explosion. The rupture disc closing the outermost end of the discharge pipe or elbow is fully and substantially instantaneously opened upon actuation of the associated electrically operated detonator. An exemplary elbow construction is shown in U.S. Pat. No. 4,394,868 of July 26, 1983 and assigned to the assignee hereof.
It has now been discovered that upon release of HALON conventional protections release tem, flow of the HALON through a discharge pipe from a storage tank takes place as two-phase turbulent flow where gas bubbles are intimately mixed with the liquid. Skilled workers in the explosion protection field have heretofore failed to adequately appreciate and recognize that this turbulent flow significantly increased the overall suppressant discharge time. In fact, many systems now being commercialized have exacerbated the problem by providing long lengths of pipes leading from remote suppressant storage vessels to a discharge orifice many feet from the containment unit. In addition, the piping arrangements frequently had a number of elbows in the delivery path which further aggravated the unrecognized turbulent flow problem.
Suppressant flow restriction and increased flow time occurs whenever any part of the initially liquid suppressant is permitted to vaporize prior to ejection from the delivery pipe or nozzle. Turbulent or choke flow thus occurs whenever gas bubbles are allowed to form in the liquid before release of the liquid from the system.
The dynamics of flashing flow regimes have been studied in depth in recent years by a number of investigators including Hans K. Fauske, Robert E. Henry, J. C. Leung and M. A. Grolmes of Fauske & Associates, Burr Ridge, Ill. Their work, conducted in part under the auspices of the American Institute of Chemical Engineers (AIChE) through a research program known as the Design Institute for Emergency Relief Systems (DIERS) demonstrated that two-phase flow can have a profound effect on the time required for an initially liquid material to be exhausted from a contained vessel. Although the DIERS study was directed to emergency relief in the case of a runaway chemical reactor, the work is instructive from the standpoint that it confirmed that flashing flows are generally always choked and turbulent in nature. Theoretical equations for such flows are described in a paper by Leung and Grolmes published in the March 1987 AIChE Journal, Vol. 33, No. 3, which references in part Leung's article in Vol. 32, No. 10 of the AIChE Journal published in 1986 and entitled "A Generalized Correlation for One-Component Homogeneous Equilibrium Flashing Choked Flow." These correlations make it clear that two-phase flow is very complex and highly dependent on a number of ambient environment parameters including temperature and pressure of the composition to be ejected, and the ratio of gas to liquid in the flow regime.